Basically, in case the Master SD is down, that function should move to the other SD on the same DC.
Taken from [1]:
To change the Master Storage Domain to another specific Storage Domain, the below steps need to be followed:
* Put all storage domains except the Master storage domain and the one that needs to be the new master storage to maintenance mode from
Data Centre -> Lower Sub tab -> Storage -> Right-click -> Maintenance.
(The only active storage domains now would be the Master storage domain and the storage domain which need to be the master.)
*Now put the "Master storage" to maintenance mode.
* The only active storage domain (which you want to make the new master storage) would automatically be the master storage.